A connection structure of a substrate device of the related art will be described with reference to FIG. 8.
Referring to FIG. 8, a transmission line 12 serving as a signal line is mounted on a printed board 11. The transmission line 12 is connected to a pad 13 for an edge connector 16 at the end of the board. The pad 13 and edge connector 16 are connected to each other at a contact point 14. A transmission signal is transmitted from the transmission line 12 to the edge connector 16 via the pad 13 and contact point 14, or transmitted in the reverse direction from the edge connector 16 to the transmission line 12 via the contact point 14 and pad 13.
The pad 13 for the edge connector 16 provided at the end of the printed board 11 needs to be designed such that it has a diameter larger than the contact point 14 in terms of manufacturing requirements. Thus, for a high-speed signal, the part of the pad 13 that is positioned on the opposite side of the transmission line 12 with respect to the contact point 14 is a wiring branched from the transmission line 12. This wiring is fundamentally unnecessary for signal transmission and is called “stub” (stub 15). The leading end of the stub 15 is not connected anywhere and is opened, so that a signal branched from the transmission line 12 is reflected at the open end to become noise. Particularly, in high-speed signal transmission, this portion may be a source of noise, resulting in degradation of signal quality (signal waveform).
The stub 15 is coupled to a ground layer 17 positioned at the lower layer of the printed board 11 to have capacitance, so that a capacitance component fundamentally unnecessary for the transmission line 12 is added thereto. This may cause impedance mismatch, leading to reflection noise. Particularly, in high-speed signal transmission, influence of the reflection noise becomes large relative to the noise margin of a signal in the voltage axis direction and time axis direction, making it difficult to secure the eye pattern in a signal waveform.
In relation to the above problem, JP-A-2008-046142 has proposed a multilayer printed board for mounting device to be measured in which a through hole is used to connect an electrode pad and inner layer wiring pattern. In this printed board, for achievement of high speed signal transmission, an SVH (Surface Buried Via Hole) is formed in one of both ends of an inner layer wiring pattern to accordingly shorten the length of a stub portion in a through hole to thereby reduce a stub capacitance.
In the technique of JP-A-2008-046142, the removal of the stub in the through hole is mechanically conducted by drilling from the back surface of the printed board, since the through hole can be formed only in a columnar shape in terms of manufacturing constraint.
On the other hand, in the case of the pad 13 for the edge connector 16 positioned at the end of the printed board 11 of FIG. 8, when the removal of the stub is mechanically conducted by drilling as in JP-A-2008-046142, a burr may remain in the pad 13, degrading reliability of connection with the contact point 14.
The present invention has been made to solve the above problems, and an object thereof is to provide a substrate device capable of reducing reflection noise from the stub end of the pad to reduce degradation of a signal waveform and improving impedance mismatch caused due to a capacitance between the pad stub and ground layer.